Effect of Sample Preparation on Microwave Material Characterization by Loaded Waveguide Technique

Microwave material characterization is an important nondestructive evaluation tool, as many physical and chemical properties can be related to a material's dielectric properties. These properties can be measured using a number of methods including the loaded waveguide technique. This method requires that a sample be placed in a waveguide sample holder and subsequently utilizes measured complex reflection (S-11) and transmission (S-21) properties to calculate the sample's dielectric properties. As such, it is important that the sample be prepared carefully, as the dielectric property calculation assumes a perfect (ideal) sample geometry. However, in practice, samples are oftentimes prepared by hand, resulting in a distorted sample geometry. This paper presents a simulation and measurement study on a number of potential sample preparation errors and the effect of these errors on calculated dielectric properties. Finally, a statistical analysis (including mean, standard deviation, coefficient of variation, and confidence interval) was applied to provide a method by which calculated dielectric properties (even when imperfect samples are used) can be checked to ensure that proper accuracy of the results has been achieved.